The present invention relates to methods of reconstructing signal amplitudes for video coding and compression. More specifically, it relates to methods for chroma deblock filtering processes in video coding and processing systems such as within the High Efficiency Video Coding (HEVC) standard.
The HEVC standard, currently published as ISO/IEC 23008-2 MPEG-H Part 2 and ITU-T H.265, introduced several new video coding tools designed to improve video coding efficiency over previous video coding standards and technologies, including, but not limited to MPEG-2, MPEG-4 Part 2, MPEG-4 AVC/H.264, VC1, and VP8.
One of the tools introduced in the HEVC standard is deblocking (also referred to as “deblock filtering”), which is a filtering mechanism performed to reduce the appearance of “blockiness” by smoothing out artifacts caused by block-wise processing. Blockiness artifacts commonly occur at block boundaries, thus deblocking modifies boundary sample values to remove blockiness artifacts. The boundary samples of adjacent coding units are smoothed to reduce the transitions between coding units. Deblock filtering under the HEVC standard is applied for both luma and chrominance (chroma) components of reconstructed video samples. Under the HEVC standard, deblock filtering processes for luma components is different from deblock filtering processes for chroma components. In particular, chroma components typically undergo a simplified deblock filtering process compared with luma components.
Under the HEVC standard deblocking scheme, luma components are subject to one type of deblock filtering, and chroma components are subject to another type of deblock filtering. Under the HEVC standard scheme for chroma deblock filtering, color artifacts, sometimes severe, may result. The color artifacts may result from differing filter strengths by which luma and chroma deblocking is applied, the sample grid sizes on which chroma deblocking is applied, and the limited modes in which chroma deblocking is applied. For instance, deblock filtering is typically applied to samples adjacent to a (PU) or a (TU) boundary, i.e., edges that are aligned on an 8×8 sample grid, for both luma and chroma samples. Thus, for chroma samples, deblock filtering may be performed on a 16×16 boundary in a typical 4:2:0 sampling structure. Additionally, chroma deblocking is performed only when one of two adjacent coding units use intra mode prediction. Furthermore, the filter size is relatively large, which provides weak filtering by only affecting one pixel at each side of an edge.
With respect to deblock filter strength, the HEVC specification defines three strength levels: no filtering, strong filtering, and weak filtering. A strong filter (i.e., strength level of 2) is applied when one of two adjacent blocks is intrapicture predicted. A weak filter (i.e., strength level of 1) is applied when P or Q has at least one nonzero transform coefficient, the reference indices of P and Q are not equal, the motion vectors of P and Q are not equal, or a difference between a motion vector component of P and Q is greater than or equal to one integer sample. Otherwise no deblock filtering is applied.
Under the HEVC standard, deblock filtering of luma components typically is applied in one of any of the three strengths described above. In contrast, deblock filtering of chroma components is typically applied in one of two strengths described above: no filtering or normal filtering (also referred to as “weak filtering”). The decision of whether to perform deblock filtering on a boundary is based on a boundary filtering strength variable, “bS.” bS is typically determined based on a prediction mode used to reconstruct a CU. For example, according to the HEVC standard, when the luma component of a block is intra code, the value of its bS is at least two. According to the filter strength and the average QP, two thresholds, tc and β, are determined from predefined tables. For luma samples, the type of filtering performed is selected based on β and tc. For chroma samples, only two types of filtering are used: no filtering or normal filtering. The strength level of the filter indicates the number of samples that are modified on each side of a boundary. For example, a normal filter might modify one to two samples on each side a boundary, while a strong filter might modify up to three samples on each side of a boundary. Thus, there exists a need in the art for improved chroma deblock filtering.